1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a mechanism for loosening a sheet stack by blowing air on an edge portion of the sheet stack.
2. Description of the Related Art
Of conventional image forming apparatuses such as copying machines and printers, there is provided an image forming apparatuses including a sheet feeding apparatus for feeding sheets one by one, starting from an uppermost sheet of the stacked sheets, toward an image forming portion while separating the sheets stacked on a sheet stacking portion.
As disclosed in Japanese Patent Application Laid-Open No. H07-196187 (U.S. Pat. No. 5,645,274), for such the sheet feeding apparatus, there exists a system for feeding sheets by blowing air (gas) on an edge portion of a sheet stack on a sheet stacking portion to allow a plurality of sheets to float and allowing an uppermost sheet of the sheet stack to suck onto a suction conveyer belt.
FIG. 6 shows a structure of a sheet feeding apparatus for feeding sheets by allowing the sheets to float using air and allowing an uppermost sheet of the floating sheets to suck onto a suction conveyer belt.
In FIG. 6, a storage 11 is provided to an image forming apparatus main body (not shown) so as to be drawable and stores a sheet S. The storage 11 is provided with a sheet tray 12 on which a plurality of sheets S are stacked and capable of moving up and down, and a trailing edge regulating plate 13 for regulating a position of a sheet trailing edge which is an edge portion on an upstream side (rear side) in a sheet feeding direction of a sheet stack SA. Further, the storage 11 is provided with side edge regulating plates 14 and 16 for regulating positions of side edges, which are edge portions in a width direction perpendicular to the sheet feeding direction of the stacked sheet S. Further, the image forming apparatus main body (not shown) is provided with slide rails 15 for drawing the storage 11.
A suction conveyer belt 21 sucks and feeds the uppermost sheet of the floating sheets, and a suction fan 36 creates a negative pressure for allowing the sheet S to suck onto the suction conveyer belt 21. An air blowing portion 30 blows air on a sheet leading edge, which is an edge portion on a downstream side (i.e., front side) in the sheet feeding direction of the sheet stack SA. The air blowing portion 30 includes a separation fan 31, a separation duct 32, a loosening nozzle 33, and a separation nozzle 34.
Here, the trailing edge regulating plate 13 can move in a direction indicated by an arrow R and the side edge regulating plates 14 and 16 can move in a direction indicated by the arrows Q shown in FIG. 7 according to a size of a sheet stacked on the sheet tray 12. In FIG. 7, a chain double-dashed line indicates a position of a sheet SL of a maximum size and a long dashed dotted line indicates a position of a sheet SS of a minimum size when the sheet SL and the sheet SS are stacked on the sheet tray 12, respectively.
Note that, the side edge regulating plates 14 and 16 need to retain sheets ranging from the sheet SS of the minimum size to the sheet SL of the maximum size in a stable manner, so the side edge regulating plates 14 and 16 are provided in a state where the plates are divided into a plurality of pieces in the sheet feeding direction. The sheet tray 12 is partially cut away in portions in which the side edge regulating plates 14 and 16 move to thereby the sheet tray 12 assumes a shape of a hatched region of FIG. 7.
Here, in the sheet feeding apparatus having the above-mentioned structure, in a case where a user draws out the storage 11 from the image forming apparatus main body to set the sheet S therein and then stores the storage 11, the sheet tray 12 moves up in a direction indicated by the arrow A shown in FIG. 8 by drive means (not shown). The sheet tray 12 stops and waits at a position, where a distance between an upper surface of the sheet stack SA and a suction surface of the suction conveyer belt 21 corresponds to a dimension B, while being ready for a sheet feeding signal.
Next, when the feeding signal is input, the separation fan 31 operates to suck air in a direction indicated by the arrows C of FIG. 9. The air sucked by the separation fan 31 passes through the separation duct 32 and is blown from the loosening nozzle 33 and the separation nozzle 34 in directions indicated by the arrows D and E, respectively, toward a leading edge portion of the sheet stack SA. The air blown from the loosening nozzle 33 allows several sheets Sa on an upper portion of the sheet stack SA to float, and the air blown from the separation nozzle 34 separates a sheet next to an uppermost sheet Sb from the sheet Sb adhering onto the suction conveyer belt 21. Meanwhile, the suction fan 36 operates to discharge the air from a suction duct 38 in a direction indicated by the arrows F of FIG. 9. At this time, a suction shutter 37 provided in the suction duct 38 is closed.
Note that, the side edge regulating plates 14 and 16 are provided with auxiliary separation fans 17 and 18, respectively. The auxiliary separation fans 17 and 18 blow air on a side edge portion of the sheet stack SA from openings 14A and 16A, respectively. The auxiliary separation fans 17 and 18 are thus provided, thereby allowing the several upper sheets Sa to reliably float. Examples of a document describing such the structure include Japanese Patent Application Laid-Open No. 2003-182873.
Next, when the several upper sheets Sa stably float after a predetermined period of time has passed since the feeding signal is input, the suction shutter 37 is rotated in a direction indicated by the arrows G of FIG. 10. As a result, a suction force is generated by the suction fan 36 in a direction indicated by the arrows H from suction holes (not shown) formed in the suction conveyer belt 21, thereby allowing the uppermost sheet Sb of the several upper sheets Sa, which are allowed to float, to be sucked by the suction conveyer belt 21.
By rotating belt drive rollers 41 in a direction indicated by the arrows J of FIG. 11, the sheet Sb being sucked by the suction conveyer belt 21 is fed in a direction indicated by the arrow K. After that, the sheet Sb is sent to a conveying path by a drawing roller pair 42 including rollers rotating in directions indicated by the arrows M and P, respectively.
In the conventional sheet feeding apparatus having a structure in which sheets can be fed in a separated state by blowing air on the sheet stack SA, in a case of using a sheet which has a low strength and is long in the sheet feeding direction, as shown in FIG. 12, a portion is generated in which the several upper sheets Sa are not allowed to float.
As shown in FIG. 12, in a region where the sheets are brought into contact with one of the side edge regulating plates 14 on the side of the air blowing portion 30 of the sheet stack SA, owing to a floating force generated by air from the air blowing portion 30 and air from the auxiliary separation fan 17, the sheets in the sheet stack SA is equally loosened.
However, in a portion between the region where the sheets are allowed to float by the auxiliary separation fan 17 and a region where the sheets are allowed to float by the auxiliary separation fan 18, there is generated a region 35B of FIG. 12 in which no sheet S is allowed to float at all. Further, on the trailing edge side of the region in which the sheet is allowed to float by the auxiliary separation fan 18, there is generated a region 35C of FIG. 12 where no sheet S is allowed to float at all.
Still further, as shown in FIG. 12, a difference shown as a dimension C is generated in a portion between a height to which the uppermost sheet is allowed to float in the region where the side edge regulating plate 14 on the right side in FIG. 12 comes into contact with the sheets and a height to which the uppermost sheet is allowed to float in the region where the side edge regulating plate 16 on the left side in FIG. 12 comes into contact with the sheets. This is because while in the region where the side edge regulating plate 14 on the right side comes into contact with the sheets, both the air blowing portion 30 and the auxiliary separation fan 17 impart the floating force to the sheets, in the region where the side edge regulating plate 16 on the left side comes into contact with the sheets, only auxiliary separation fan 18 imparts the floating force. In this case, the region where no sheet S is allowed to float is a region where the sheets adhere to each other. If such the region exists, in feeding the sheets by the suction conveyer belt 21, it is impossible to prevent generation of a double feed due to the adhesion of the sheets.
FIG. 13 is a top view showing flows of air at this time. In FIG. 13, a hatched portion indicates an example of a portion where adhesion of the sheets is not eliminated. That is, the hatched portion corresponds to a portion where the sheets are adhered to each other. Further, in FIG. 13, portions indicated by circles are portions where air leaks laterally to outside. When air leaks laterally to the outside as shown in FIG. 13, air to be blown passes the sheets without allowing the sheets to float, so the sheets cannot be sufficiently loosened.
In particular, when the sheet S is a so-called coated paper having a surface coated with a coating material, which is used for printing in many cases, it is not uncommon that a suction force increases 10 N or more owing to temperature or humidity of the use environment. Thus, when using such the sheet, in the case where the sheets cannot be sufficiently loosened by blowing air, there arise not only a problem in that the sheets are adhered to each other to be double fed, but also sometimes a problem in that ten or more sheets are fed together, thereby causing a jam.